1. Field of the Invention
The present invention relates generally to specimen inspection. More particularly, the present invention relates to e-beam inspection systems.
2. Description of the Background Art
An example of an electron beam (e-beam) inspection system is shown in FIG. 1 for purposes of background explanation. The secondary electron emission microscope (SEEM) system of FIG. 1 is a projection type system, where a large spot of electrons rather than a small one is formed at the surface of the specimen, and the secondary electrons from this spot are imaged onto a two-dimensional detector. Typically, the specimen may comprise a semiconductor wafer having integrated circuit related structures formed on its surface. Alternatively, the specimen may be another type of sample.
The system of FIG. 1 is described in U.S. Pat. No. 5,973,323, entitled “Apparatus and Method for Secondary Electron Emission Microscope,” inventors Adler et al., and assigned at issuance to KLA-Tencor Corporation of San Jose, Calif. The disclosure of U.S. Pat. No. 5,973,323 is hereby incorporated by reference. As described in that patent, FIG. 1 shows the basic configuration for the Secondary Electron Emission Microscopy (SEEM) apparatus. An electron gun source 10 emits a beam 11 of primary electrons e1 along path 12. The electron beam 11 is collimated by electron lens 13 and continues along path 12. Magnetic beam separator 14 then bends the collimated electron beam 11 to be incident along electron optical axis OA normal to the surface to be inspected. Objective electron lens 15 focuses the primary electrons, e1, into a beam having a spot size typically in the range 1-10 mm and an incident energy on the order of 1 keV on specimen 9.
Primary electrons e1 incident on the specimen 9 produce secondary electrons e2 which travel back along the axis OA perpendicular to the inspection surface to objective electron lens 15, where they are re-collimated. Magnetic beam separator 14 bends the electrons to travel along image path 16. The electron beam along image path 16 is focused by projection electron lens 17 to image plane 18, where there is an electron detector 19, which is a camera or preferably a time delay integrating (TDI) electron detector. The operation of an analogous TDI optical detector is disclosed in U.S. Pat. No. 4,877,326, entitled “Method and Apparatus for Optical Inspection of Substrates,” inventors Chadwick et al., and assigned at issuance to KLA Instruments Corporation. The disclosure of U.S. Pat. No. 4,877,326 is incorporated herein by reference. The image information may be processed directly from a ‘back thin’ TDI electron detector 19, or the electron beam may be converted into a light beam and detected with an optional optical system 20 and a TDI optical detector.
FIG. 2 shows parallel imaging in the Secondary Electron Emission Microscopy inspection technique. Beam 54 is produced from an electron gun source, and beam 54 has a width “W,” typically about one to two millimeters, at the surface of sample 55. Sample 55 has the characteristic dimension D, which is much greater than the width W of the electron beam. In SEEM, the width of the electron beam 54 is much larger than in secondary electron microscopy (SEM), but it may still be necessary to move the sample 55 with respect to the beam to scan the sample 55. However, in the preferred embodiment, SEEM requires only mechanical movement of the stage of the sample 55 with respect to beam 54, and not an electron beam deflection system for electromagnetically steering beam 41. The SEEM inspection system can operate much faster than the SEM inspection system because SEEM images thousands or millions of pixels in parallel.
FIG. 2 further shows a magnified view of the imaging portion of the beam 54 on the sample 55 to illustrate the parallel, multiple pixel imaging region 56 within beam 54. A rectangular detector array region 56 occupies a central portion of the beam 54 and defines the imaging aperture. (The detector array is either of the time delay integrating (TDI) or non-integrating type.) The detector array 56 may, for example, image between about 500 thousand and one million pixels in parallel.
Despite advances in e-beam inspection, such as SEEM described above, further improvement may be made. For example, it is typically desirable to increase the throughput of an inspection system (the rate at which specimens may be inspected by the system). Factors that limit the throughput of an e-beam inspection system include the usable size and intensity of the beam at the specimen plane. Generally, the larger the usable size of the beam and the higher the usable intensity of the beam, the higher the potential throughput.